A Two-Dimensional Lumbar Load Estimation Model for Comparing Conceptual Assist Mechanisms

Lumbar assist suits are commonly evaluated based on reductions in muscle activity; however, such reductions do not necessarily correspond to decreases in intervertebral loading. This study developed a two-dimensional sagittal-plane static rigid-link model to evaluate relative changes in lumbar intervertebral loads during assist suit use. The model incorporated subject-specific spinal alignment based on age, sex, and height and estimated compressive force, shear force, and external flexion moment at each intervertebral level from L1/L2 to L5/S1. Three conceptual assist conditions—shoulder-traction assistance, extensor-muscle assistance, and lumbar stabilization—were compared at L4/L5 and L5/S1. The assist parameters were calibrated to reduce the L5/S1 ES-group force by 20% at 60° trunk flexion. Under shoulder-traction assistance, compressive force, shear force, and external flexion moment decreased at both L4/L5 and L5/S1. Under extensor-muscle assistance, erector spinae force decreased as prescribed, whereas the three intervertebral load components remained unchanged. Lumbar stabilization reduced compressive force and external flexion moment but slightly increased shear force. These findings demonstrate that similar reductions in erector spinae force can produce different changes in intervertebral loads depending on the mechanical assistance principle, highlighting the importance of considering multiple load components when evaluating and designing lumbar assist systems.